1. Field of the Invention
The present invention generally relates to digital modulation and demodulation, the former modulating a data stream into a bit stream suitable for recording or transmitting data, and the latter demodulating a bit stream regenerated from a recording medium or a transmission system into a data stream.
2. Description of the Related Art
A data stream composed of 1's and 0's is modulated into a bit stream conformable to the characteristics of an applicable recording medium, recording head, transmission medium, etc. Then, this bit stream is recorded into the recording medium or sent out to an applicable transmission system. For example, a data stream may be RLL-encoded, then NRZI-modulated, and then recorded into the recording medium. This way of recording can enhance the recording density. A data stream may also be NRZ- or NRZI-modulated, and then recorded in the recording medium.
In the RLL-coding, data words are cut out m-bit by m-bit from the input data stream, and each data word is converted into an n bit codeword. Restrictions are imposed on this RLL coding to increase the minimum value Tmin and to decrease the maximum value Tmax of time interval between adjacent transition. Specifically, restrictions are imposed so that the number of bits "0" existing between two bits "1" to be d or more and k or less. The codes to fulfill these restrictions are called "(d, k; m, n) RLL codes."
In the NRZI modulation, the RLL code is inverted by a bit "1" but not inverted by a bit "0." Thus, the bit interval between adjacent transitions in the bit stream after NRZI modulation is longer than that of the RLL code before NRZI modulation. For this reason, in comparison with a case where the bit stream before NRZI modulation is recorded in a recording medium and then regenerated, the waveform deterioration of the regenerated signal is smaller in a case where the bit stream after NRZI modulation is recorded into a recording medium and then regenerated. As a result, there is less frequent occurrence of read error. In other words, if nearly the same read error is permitted, the recording density of the case where a bit stream after NRZI modulation is recorded is higher than the case where a bit stream before NRZI modulation is recorded
The bit stream for recording or transmitting data should preferably have the following characteristics:
(1) Tmin: The minimum time interval between adjacent transitions
Tmin is the product of "d+1" and Tw, where Tw is the duration of a channel bit. When the recording density is enhanced, the duration of the channel bit stream is shortened, and as a result, the regenerated signal may be easily deteriorated by inter-symbol interference. Thus, read errors may easily occur. Therefore, to reduce the inter-symbol interference that may occur when the bit stream is read from a recording medium with a high recording density, the Tmin should preferably be large.
(2) Tmax: The maximum time interval between adjacent transitions
Tmax is the product of "k+1" and Tw. Unless the transition occurs, no regenerated pulse can be obtained, and for this reason, the edge of the clock can not be positioned accurately at the center of the channel bit stream. If the same bit is generated for a long duration, a DC component is largely contained in a channel bit stream. Therefore, Tmax should preferably be small.
(3) DC or low-frequency component
A recorder for recording a bit stream into a recording medium and a regenerator for regenerating a signal recorded in a recording medium have AC coupling elements. A unit for converting a bit stream to an analog signal and sending out the same to a transmission system and a unit for regenerating an analog signal received from a transmission system also have AC coupling elements. Thus, when the analog recording or sending signal produced from bit streams has a DC component, the AC coupling elements deteriorate the waveform of this signal, which is not preferable. Furthermore, the DC component lost due to this deterioration cannot be restored. Therefore, it is preferable that the DC or low-frequency component should be small. For evaluating the DC or low-frequency component of the recording signal, a DSV (Digital Sum Value) is used. The DSV takes a bit "1" as "+1" and a bit "0" as "-1," and obtains the accumulated value of a recording bit stream from the head thereof. The smaller the absolute value of the DSV is, the smaller is the DC or low-frequency component. For the evaluation of the DC or low-frequency component in each code word within the recording signal, a CDS (Codeword Digital Sum) is used. The CDS is the DSV within each code word. The smaller is the CDS, the smaller is the DC or low-frequency component of code word.
(4) Tw: Duration of channel bit (window margin)
The detection window margin Tw is given by (m/n)T, where T is the bit interval of the data stream before modulation. The detection window margin TW indicates the time available to the detection of the channel bit, i.e., resolution. It also indicates the tolerance of the phase variation of the regenerated signal due to inter symbol interference, noise or the like. It is preferable that Tw should be large.
(5) Lc: Constraint length
Encoding may be made by referring to the preset codewords and the future codewords as well as the current codeword to improve Tmin, Tmax and the DSV. Here, the length of the codewords to be referred to is called a "constraint length Lc." The larger Lc is, the larger the propagation of error during demodulation is, and the more complicated the circuitry is. For this reason, it is preferable that LC should be small.
The Japanese publication No.52-128024 of unexamined patent application discloses a technique to make Tmin of the recording bit stream after NRZI modulation larger and Tmax of the same smaller. According to this publication, a (1, 7; 2, 3) RLL code table is used by the RLL coding, i.e., by cutting out a 2-bit data word from the input data stream and translating it into a 3-bit codeword, respectively. The bit stream of the RLL code thus generated is NRZI-modulated. If restriction of d=1 fails to be satisfied, a (1, 7; 4, 6) RLL code table is used.
The Japanese publication No. 1-27510 of examined application discloses a technique to reduce the DC component of the channel bit stream, i.e., an RLL coding technique, to prevent Tmin of the channel bit stream from being reduced. According to this publication, n-bit codewords are cut one-by-one from the bit stream after RLL encoding and a plurality of redundant bits are inserted into each space between the adjoining codewords, and the bit stream inserted with these redundant bits is supplied to the NRZI modulation circuit. These redundant bits are selected based on the necessity of inversion of the codeword before which the redundant bits are to be inserted and also on the state of the tail-end part of the codeword immediately before the redundant bits. That is, the selection is so made that the DC component of the channel bit stream can be reduced and Tmin can be prevented from being reduced.
The Japanese publication No. 5-34747 of examined application discloses a code conversion method which enables Tmin to be 1.5 T, Tmax to be 4.5 T and Lc to be 5 T by adjusting the conversion rules for translating the data stream into an RLL code stream according to the arrangement of the data stream.
The Japanese publication No. 4-77991 of examined application discloses a technique to reduce the DC components of the channel bit stream and increase Tmin. According to this publication, 8-bit data words are cut out one by one from the input data stream, and each data word is translated into a 14-bit code word. In this conversion, the number of bits "0" between the bit "1" and the bit "1" is 1 or more and 8 or less in the translated bit stream. Two different tables for translating 8-bit data words into 14-bit code words are available. The codeword of either of these tables is selected according to the DSV at the end of the codeword translated immediately before. That is, the selection is made so that the DC components of the channel bit stream can be reduced.
The Japanese publication No. 6-311042 of unexamined application discloses a technique for sufficiently reducing the DC components of the channel bit stream and improve the recording density ratio (DR) by increasing Tmin. According to this publication, the 8-bit data words are cut out one-by-one from the input data stream, and each data word is translated into a 17-bit codeword. In this conversion, the number of bits "0" between the bit "1" and the bit "1" is two or more and nine or less in the translated bit stream. The above 17-bit codeword can be obtained by multiplexing a 2-bit redundant bit to the 15-bit code word corresponding to the 8-bit data word. According to the Japanese Unexamined Patent Publication No. 6-311042, two different tables for corresponding the 8-bit data word to the 15-bit codeword are available, and 3 different 2-bit redundant bits are also available. From among the six different code words obtained by combining these two different tables and three different redundant bits, the above 8-bit data word is converted to the 17-bit code word selected according to the DSV at the end of the data translated immediately before. That is, the above 8-bit data word is converted to the 17-bit code word selected so as to reduce the DC components of the channel bit.